Pyruvate Dehydrogenase Kinase (PDK)
Manipulating major cellular processes at individual key regulatory points may provide a relatively simple means to affect several phenotypes. Respiration and lipid biosynthesis for example may be simultaneously modified by altering levels of acetyl-CoA which both serve as entry point into the Krebs cycle as well as primary substrate for fatty acid biosynthesis. Increased respiration may be manifested in an increase in seed growth as in soybean (Sinclair et al. (1987) Plant Physiol 83:467-468) whereas decreased respiration may lead to decreased reproductive growth (Gale (1974) J Exp Bot 25:987-989).
The pyruvate dehydrogenase enzyme complex catalyzes the oxidative phosphorylation of pyruvate into acetyl-CoA. The multienzyme complex is composed of many different enzymes that catalyze different reactions in forming acetyl-CoA. Pyruvate dehydrogenase (E1) is a thiamine pyrophosphate (TPP)-requiring enzyme that decarboxylates pyruvate with the formation of hydroxyethyl-TPP. The hydroxyethyl group attacks the disulfide bond of the lipoamide moiety of the second enzyme, dihydrolipoyl transacetylase (E2) to form acetyl-dihydrolipoamide-E2 and regenerate E1. E2 then catalyzes the transfer of the acetyl group to CoA, forming acetyl-CoA and dihydro-lipoamide-E2. Dihydrolipoyl dehydrogenase (E3) via its flavin adenine dinucleotide (FAD) group oxidizes the dihydrolipoamide moiety linked to E2, regenerating lipoamide-E2. Reduced E3 is then reoxidized by NAD+, forming NADH. In eukaryotes, the complex is composed of 30 E1 dimers and 6 E3 dimers around a core of 60 E2 monomers arranged in a dodecahedron.
One mechanism of regulating pyruvate dehydrogenase activity is its phosphorylation state. The enzyme pyruvate dehydrogenase kinase or PDK inactivates the E1 subunit by catalyzing the phosphorylation of a specific E1 Ser residue using ATP. Hydrolysis of this phospho-Ser residue by the pyruvate dehydrogenase phosphatase reactivates the complex. This form of regulation operates only in mitochondria and not in chloroplasts. Suppressing pyruvate dehydrogenase activity may therefore lead to increased mitochondrial pyruvate dehydrogenase activity leading to increased respiration and fatty acid and hence oil biosynthesis. Suppression of pyruvate dehydrogenase kinase activity may be accomplished by downregulating expression of genes encoding pyruvate dehydrogenase kinase by technology well known to those skilled in the art which include antisense inhibition and cosuppression. Indeed, WO 98/35044 describes transgenic Arabidopsis thaliana plants transformed with antisense constructs of the Arabidopsis pyruvate dehydrogenase kinase gene as having increased pyruvate dehydrogenase activity, increased activity of enzymes involved in the Krebs cycle, increased overall oil content, and shorter flowering time as brought about by increased respiration, without any apparent deleterious effects. Nucleic acid fragments encoding pyruvate dehydrogenase kinase have also been isolated from maize (Thelen et al. (1998) J Biol Chem 273:26618-26623) The domains responsible for catalysis and recognition and binding of substrates remain to be defined. Accordingly, the availability of nucleic acid sequences encoding all or a portion of pyruvate dehydrogenase kinase would facilitate studies that address these issues and could provide genetic tools to enhance or otherwise alter the accumulation of carbohydrates, lipids and proteins in plants and seeds.
Dihydrolipoamide Dehydrogenase
Carbon flux in living organisms is governed by intricate metabolic pathways linked together by regulatory mechanisms that take their cue from a variety of signals including but not limited to the environment, the developmental stage, genetics, and physiology. It is becoming common practice to alter accumulation of certain metabolites by manipulating expression of a key enzyme in the relevant pathway. For example, U.S. Pat. No. 5,773,691 describes a method of increasing the lysine content of seeds by overexpressing in a seed-specific manner a gene encoding the enzyme dihydrodipicolinic acid synthase, a major regulatory point in lysine biosynthesis.
An enzyme that is part of a number of several metabolic pathways is dihydrolipoamide dehydrogenase. Dihydrolipoamide dehydrogenase is a flavoprotein that catalyzes the oxidation of dihydrolipoyl moieties of noncovalently associated proteins in multienzyme complexes including the pyruvate dehydrogenase complex, α-ketoglutarate dehydrogenase complex, and branched-chain α-ketoacid dehydrogenase complex, where it is referred to as the E3 component, because it is the third enzyme in the reaction mechanism. Dihydrolipoamide dehydrogenase has also been shown to be a component of the glycine cleavage system, where it is referred to as the L protein. These E3-dependent enzyme complexes catalyze key regulatory reactions in intermediary metabolism, including plant leaf respiration. In plants, the plastid form of the pyruvate dehydrogenase complex provides acetyl-CoA and NADH for fatty acid biosynthesis. The importance of this enzyme is demonstrated by the fact that mice that have both copies of the gene encoding dihydrolipoamide dehydrogenase inactivated die prenatally (Johnson et al. (1997) Proc Natl Acad Sci USA 94:14512-14517).
Genes encoding dihydrolipoamide dehydrogenase have been isolated from prokaryotes (Stephens et al. (1983) Eur J Biochem 135:519-527), yeast (Ross et al. (1988) Gen Microbiol 134:1131-1139) and animals (Johnson et al. (1997) Genomics 41:320-326). A gene encoding the mitochondrial enzyme in pea leaves has been isolated. It is believed that a single nuclear gene encodes the same mitochondrial dihydrolipoamide dehydrogenase in the pyruvate dehydrogenase and glycine decarboxylase complexes (Turner et al (1992) J Biol Chem 267:7745-7750; Bourguignon et al. (1996) Biochem J313 229-234). The plastidic counterpart has been biochemically characterized, and appears to be distinct from the mitochondrial enzyme. They appear to share limited sequence similarity, and antibodies to the mitochondrial enzyme did not readily cross-react with its plastidic counterpart (Conner et al. (1996) Planta 200:195-202). The gene encoding the plastidic dihydrolipoamide dehydrogenase remains to be isolated. Accordingly, the availability of nucleic acid sequences encoding novel amino acid sequences of dihydrolipoamide dehydrogenase would facilitate studies to better understand carbon flux in plants and could provide genetic tools to enhance or otherwise alter the accumulation of particular metabolites like fatty acids during plant growth and development.
Steroid Dehydrogenase
Steroids constitute an integral component of plant membranes. They decrease fluidity and probably are involved in the adaptation of membranes to temperature. More recently, the importance of steroids as plant hormones has been the subject of intense study, stemming from the discovery that Arabidopsis de-etiolated (det2) and the constitutive photomorphogenesis and dwarfism (cpd) mutants are defective in the synthesis of brassinosteroids (Li et al. (1996) Science 272:398-401; Szekeres et al. (1996) Cell 85:171-182).
Structurally similar to animal steroids, brassinosteroids have been shown to elicit a broad spectrum of responses including stem elongation, inhibition of root growth, repression of stress-regulated genes, pollen-tube growth and xylem differentiation (Rouleau et al. (1999) J Biol Chem 274:20925-20930; Schumacher and Chory (2000) Curr Opin Plant Biol 3:79-84).
3-beta-hydroxy-delta(5)-steroid dehydrogenase (EC 1.1.1.145) is also called progesterone reductase. It is an oxidoreductase which acts on the CH—OH group of donors with NAD+ or NADP+ as acceptors in the C-21 steroid metabolism and the androgen and estrogen metabolisms. The enzyme converts 3 beta hydroxy-5-ene-steroids into 3-keto-4-ene derivatives and interconverts 3 beta-hydroxy and 3-keto-5 alpha-androstane steroids (Labrie et al. (1992) J. Steroid Biochem. Mol. Biol. 41:421-435). This enzyme is essential for the biosynthesis of all active steroid hormones (Payne et al. (1997) Steroids 62:169-175). Levels of steroid dehydrogenases may therefore be altered to control steroid hormone biosynthesis and responses in living matter, including enhanced biomass production as seen in transgenic plants overexpressing DET2.
Plant Homologs of Yeast RFT1
Cells divide by duplicating their chromosomes and segregating one copy of each duplicated chromosome, as well as providing essential organelles, to each of two daughter cells. Regulation of cell division is critical for the normal development of multicellular organisms. A cell that is destined to grow and divide must pass through specific phases of a cell cycle: G1, S (period of DNA synthesis), G2, and M (mitosis). Studies have shown that cell division is controlled via the regulation of two critical events during the cell cycle: initiation of DNA synthesis and the initiation of mitosis. Several kinase proteins control cell cycle progression through these events. These protein kinases are heterodimeric proteins, having a cyclin-dependent kinase (Cdk) subunit and a cyclin subunit that provides the regulatory specificity to the heterodimeric protein. These heterodimeric proteins regulate cell cycle by interacting with proteins involved in the initiation of DNA synthesis and mitosis and phosphorylating them at specific regulatory sites, activating some and inactivating others. The cyclin subunit concentration varies in phase with cell cycle while the concentration of the Cdks remain relatively constant throughout the cell cycle.
Cells with damaged DNA become arrested in G1 and G2 while the damage is repaired. The p53 protein is involved in this inhibition. The p53 protein is a trans-activator protein that acts to regulate cellular division by controlling a set of genes required for this process (Koerty et al. (1995) J. Biol. Chem. 270(38):22556-22564). Upon DNA damage p53 concentrations increase which stimulates the expression of a cyclin-dependent kinase inhibitor (Kasiae et al. (1991) Cell 71:587-597). This protein inhibits the activity of G1 Cdk-cyclin complexes which in turn inhibits cell cycle progression (Hollstein et al (1991) Science 253:49-53 and Koerty et al. (1995) J. Biol. Chem. 270(38):22556-22564).
The p53 gene is under intense investigation by many labs involved in mammalian cell biology, however p53 homologues have not been identified in plants. In yeast, mutations in the RFT1 gene result in defective cell cycle progression. Recent genetic and biochemical studies indicate that wild type human p53 can suppress RFT1 mutations and that the RFT1 gene product interacts physically with p53. The work suggests that the RFT1 protein may represent a novel p53 binding factor yet to be identified from mammalian cells (Koerty et al. (1995) J. Biol. Chem. 270(38):22556-22564).
There is a great deal of interest in identifying the genes that encode cell cycle regulatory proteins including p53-associated proteins in plants. These genes may be used to express cell cycle regulatory proteins in plant cells to control cell cycle, modulate cell division and possibly enhance cell tissue culture growth. Accordingly, the availability of nucleic acid sequences encoding all or a portion of a cell cycle regulatory protein would facilitate studies to better understand cell cycle regulation in plants, provide genetic tools to enhance cell growth in tissue culture. Cell cycle regulatory proteins may also provide targets to facilitate design and/or identification of inhibitors of cell cycle regulatory proteins that may be useful as herbicides.
Phosphoinositide Binding Proteins
Phosphatidylinositol transfer proteins (PITPs) belong to a broad class of lipid transfer proteins that are able to transfer lipids between membranes in vitro. Specifically, PITPs are able to transfer phosphatidylinositol and in some cases phosphatidylcholine (PC). They have been described in microorganisms, animals, and plants. Interestingly, PITPs diverge in amino acid sequence. The PITP encoded by SEC14 in Saccharomyces cerevisiae does not have sequence similarity with the mammalian PITPs, and show only limited homology with the soybean and Arabidopsis PITPs. PITPs from different organisms may substitute for one another in vitro assays or in complementation studies but appear to vary in biological function. For example, the SEC14 gene product in Saccharomyces cerevisiae is essential for survival, involved in protein exit from the yeast Golgi complex. It regulates the nucleotide pathway of PC biosynthesis by binding PC when PC level in the membrane is high resulting in the inhibition of CTP cytidylyltransferase, an enzyme in PC biosynthesis in yeast Golgi membranes. This maintains a critical diacylglycerol pool required for Golgi secretory function. Meanwhile, the SEC14 protein in the dimorphic yeast Yarrowia lipolytica is not essential for viability but required for differentiation to the mycelial form but does not appear to play a role in PC biosynthesis (Lopez et al. (1994) J Cell Biol 125:113-127). In mammalian systems, PITP has been shown to be required for epidermal growth factor signalling (Kauffmann-Zeh, et al. (1995) Science 268:1188-1190) and to participate in secretory vesicle formation (Ohashi et al. (1995) Nature 377:544-547).
Plant PITPs have been studied in less detail. More recently, two soybean proteins, Ssh1p and Ssh2p that have been identified by their ability to rescue PITP-deficient Saccharomyces cerevisiae strains were shown to exhibit biochemical properties different from those of known PITPs. Ssh1p has neither PI-transfer activity nor PC-transfer activity, whereas Ssh2p has PI-transfer activity but no accompanying PC-transfer activity. Both however have high affinity to phosphoinositides, unlike SEC14. Moreover, Ssh1p may function as a component of a stress response pathway that leads to protection from osmotic insult (Kearns et al. (1998) EMBO J 17:4004-4017). An Arabidopsis cDNA that complements the sec14 mutation has been isolated and was found to encode a protein that has homology to the SEC14 protein. The encoded protein has been shown capable of transferring PI but not PC, like Ssh2p. Its biological role remains to be determined (Jouannic et al. (1998) Eur J Biochem 258:402-410).
Isolation of more plant phospholipid transfer proteins and phospholipid-binding proteins should allow further characterization of their structure and function, and the generation of transgenic plants that exploit their utility (e.g., engineering transgenic plants with increased tolerance to abiotic stress or with more efficient lipid and/or protein transport using these proteins).
Peroxisomal Lipid Transfer Proteins
Peroxisomes are spherical cell organelles delimited by a unit membrane where metabolic pathways that produce toxic metabolites are localized, thus preventing the spread of harmful substances in the cell. The β-oxidation pathway of lipid catabolism and photorespiration both occur in peroxisomes. During the conversion of fatty acids to succinate via β-oxidation, hydrogen peroxide and glyoxylate are generated. During photorespiration, phosphoglycolate that is produced from oxidation of ribulose1,5-bisphosphate is dephosphorylated, and then transported to peroxisomes where it is further oxidized to hydrogen peroxide and glyoxylate. In the peroxisome, hydrogen peroxide is neutralized by catalase, while glyoxylate is transamidated to produce glycine. The lethality of genetic disorders such as Zellweger syndrome in humans wherein peroxisomes fail to assemble underscores the importance of peroxisomes. Zellweger patients have impaired plasmalogen and bile acid synthesis, and catabolize phytanic acid and very long fatty acids.
Lipid transfer proteins in peroxisomes have not been examined in much detail. Peroxisomal nonspecific lipid transfer proteins are small basic polypeptides that are able to transfer phospholipids and sterols between membranes in vitro. Consequently, they are believed to facilitate movement of said molecules within cells. Genes encoding these proteins have been isolated from fungi and animals, but not yet from plants. In Candida tropicalis, a novel peroxisomal nonspecific lipid transfer protein is thought to have a role in regulating β-oxidation (Tan et al. (1990) Eur J Biochem 190:107-112). A cDNA encoding human nonspecific lipid transfer protein (or sterol carrier protein 2 [SCP2]) with a peroxisome targeting sequence was shown to enhance progestin synthesis, lending support to the notion that SCP2 is involved in regulating steroid hormone synthesis (Yamamoto et al. (1991) Proc Natl Acad Sci USA 88:463-467). Overexpressing SCP2 in rat hepatoma cells resulted in increase in cholesterol content of the plasma membrane, a finding consistent with the proposed function of SCP2 in the rapid movement of newly synthesized cholesterol to the plasma membrane (Baum et al. (1997) J Biol Chem 272:6490-6498). Less is known about similar proteins in plants. Accordingly, the availability of nucleic acid sequences encoding all or a portion of peroxisome lipid transfer proteins would facilitate studies to better understand the mechanism of peroxisome function as well as lipid transport and the various pathways involving lipids (e.g., β-oxidation pathway) in plant peroxisomes and could provide genetic tools to enhance or otherwise alter the accumulation of macromolecules particularly lipids during plant growth and development.
RNA Polymerase II Subunit RPB9
Improvement of crop plants for a variety of traits, including disease and pest resistance, and grain quality improvements such as oil, starch or protein composition, can be achieved by introducing new or modified genes (transgenes) into the plant genome. Transcriptional activation of genes, including transgenes, is in general controlled by the promoter through a complex set of protein/DNA and protein/protein interactions. Promoters can impart patterns of expression that are either constitutive or limited to specific tissues or times during development.
In eukaryotic cells, genes encoding messenger RNA are transcribed by RNA polymerase II. Efficient transcription in eukaryotes is dependent upon the interaction of several polypeptides that comprise the basal transcriptional apparatus. Accurate initiation of transcription of class II genes depends upon assembly these peptides into the basal transcriptional complex containing RNA polymerase II and the general transcription factors (GTFs): TFIIA, TFIIB, TFIID, TFIIE, TFIIF, and TFIIH. Additionally, activator, coactivator and repressor proteins interact with the basal apparatus to regulate gene expression.
RNA polymerase II has long been known to be a large multimeric protein complex. Twelve of the polypeptides in the basal apparatus tightly assemble to form RNA polymerase II. The twelve polypeptides of RNA polymerase are: RPB 1-9, RPB10α, RPB10β and RPB11. The role of several of these peptides has been elucidated in a number of systems, including humans, Drosophila melanogaster and Saccharomyces cerevisiae. The role of RPB9 in plants is not known.
RPB9 is a member of the RNA polymerase II complex. In Saccharomyces, it is one of only two subunits of RNA polymerase II not essential for cell viability. Deletion of RPB9 does not prevent formation of RNA polymerase II by the other 11 subunits, however, accurate transcriptional start site selection by RPB9-deficient RNA polymerase II is abrogated, causing transcription to initiate upstream from the correct start site. RPB9 appears to recognize DNA arrest sites and may transmit signals to the elongation ternary complex affecting the efficiency of RNA polymerase II elongation. Genetic analysis in yeast has identified the general transcription factor TFIIB as well as RPB9 as important in accurate transcriptional start site selection. Mutations in RPB9 suppress the downstream shift in start site selection caused by mutations in TFIIB. Thus TFIIB and RPB9 may functionally interact and this interaction may play an important role for efficient and accurate start site selection.
The instant invention concerns the identification and isolation of RPB9 in plants. The RPB9 subunit of RNA polymerase has been cloned from rice and several RPB9 subunit genes from other plants have been identified and isolated. Because most of the regulation of gene expression in eukaryotes occurs at the level of transcription, isolation of complete RNA polymerase II complexes would facilitate studies to better understand the interplay of the various polypeptides in the basal complex and the mechanisms that control transcription in plants. Thus, RPB9 can be used as a valuable tool to isolate complete RNA polymerase II from plant extracts. It may be possible to use RBP9 to gain an understanding of transcription in plants which will permit us to exploit this process and enhance our ability to manipulate target transgenes of interest in plants.
Transcription Factor IIA (TFIIA)
TFIIA is an important component of the basal transcription machinery of RNA polymerase II which is involved in mRNA synthesis. It functions at core promoters by serving to stabilize the interaction between the TATA promoter element with the TATA-binding protein (TBP) component of TFIID, another key component of the basal transcription machinery (Buratowski et al. (1989) Cell 56:549-561; Geiger et al. (1996) Science 272:830-836). TFIIA also appears to have activator-dependent functions, since TFIIA has been shown to interact directly with activators, and that these interactions correlate with the ability to enhance TFIID-TFIIA-promoter ternary complex assembly required for transcription initiation by RNA polymerase II (Kobayashi et al. (1995) Mol Cell Biol 15:6465-6473). Using a yeast strain that contains a TBP defective for interaction with TFIIA, TFIIA activator-dependent and core promoter functions were demonstrated in vivo (Stargell et al. (2000) J Biol Chem 275:12374-12380).
In yeast, TFIIA is composed of a large subunit of 32 kilodaltons encoded by the TOA1 gene, and a small subunit of 13.5 kilodaltons encoded by the TOA2 gene. Both genes have been cloned and neither shows obvious sequence similarity with each other (Ranish et al. (1992) Science 255:1127-1129). Both TOA1 and TOA2 genes are essential for growth of yeast, indicating the importance of TFIIA.
Ethylene Responsive Element Binding Protein (EREBP)
Ethylene induction of transcription of certain ethylene-inducible pathogenesis-related protein genes has been shown to be based on the presence of the GCC box in the promoter, also known as the ethylene-responsive element (ERE), an 11-bp sequence that is able to enhance ethylene-dependent transcription from a truncated 35S promoter of cauliflower mosaic virus (CaMV) (Ohme-Takagi and Shinshi (1995) Plant Cell 7:173-182). cDNAs encoding DNA-binding proteins that specifically bind the GCC box have been isolated, and their protein products designated ERE binding proteins, or EREBPs, are characterized by a DNA-binding domain called the AP2 domain (Okamuro et al. (1997) Proc Natl Acad Sci 94:7076-7081). Although the isolated EREBP genes exhibited different patterns of expression, all were shown to be inducible by ethylene in leaves, suggesting that altering EREBP expression may be a viable strategy to engineer plant response to ethylene, pathogen, and stress.
AC-Rich Binding Factor (ACBF)
A study of the bean phenylalanine ammonia-lyase (PAL) gene promoter revealed the presence of positive and negative regulatory cis elements, including an AC-rich motif implicated in xylem expression (Seguin et al. (1997) Plant Mol Biol 35:281-291). A factor, named AC-rich binding factor (ACBF) was shown to specifically bind the AC-rich motif. The deduced amino acid sequence of ACBF contained a long repeat of glutamine residues characteristic of previously analyzed transcription factors. A heptamer of the AC-rich sequence was shown to drive xylem-specific expression of a minimal CaMV 35S promoter (Seguin et al. (1997) Plant Mol Biol 35:281-291), suggesting that by modulating ACBF expression, chimeric genes driven by promoter sequences with AC-rich sequence (ACBF binding site) may be expressed at optimal levels in a xylem-specific pattern. Also, ACBF expression levels may be engineered to regulate PAL levels, in the process regulating disease resistance response (since PAL is a key enzyme in the phenylpropanoid pathway which produces several defense-related metabolites) and isoflavone synthesis.
YABBY Transcription Factors
Flower development is a complex process fine-tuned to environmental cues that involves transition from vegetative state to reproductive development and the actual differentiation of the floral meristem into the different floral organs at predetermined positions. During the past several years, major advances have been made towards defining the process at the molecular level. Several mutants in Arabidopsis, snapdragon, and other plant species with impaired floral development have been characterized, and the corresponding genes cloned, including PLENA (PLE)/AGAMOUS (AG) (Yanofsky et al. (1990) Nature 346:35-39; Bradley et al. (1993) Cell 72:85-95), DEFICIENS (DEF)/APETALA3 (AP3) (Sommer et al. (1990) EMBO J 9:605-613; Jack et al. (1992) Cell 68:683-697), and GLOBOSA (GLO)/PISTILLATA (PI) (Trobner et al. (1992) EMBO J. 11:4693-4704; Goto and Meyerowitz (1994) Genes Dev 8:1548-1560). Most of these floral organ identity genes, whose presence or absence of expression determines what a particular whorl of floral organs would develop into, encode transcription factors, indicating a major role of transcriptional regulation in flower development. The above-mentioned genes for example belong to a family that encodes proteins with an amino-terminal DNA-binding and dimerization domain called the MADS domain, after the initials of the first four members of this gene family, MCM1, AG, DEF, and SRF (Schwarz-Sommer et al. (1990) Science 250:931-936).
Another family of transcription factors involved in floral development and meristem formation is the YABBY gene family, to which the genes FILAMENTOUS FLOWER (FIL) and CRABS CLAW (CRC) belong. These genes specify abaxial cell fate which is incompatible with a meristematic state (Siegfried et al. (1999) Development 126:4117-4128) in above ground lateral organs including leaves and flowers. Fil mutants generate underdeveloped flowers that fail to form receptacles and floral organs, and flowers with altered number and shape of floral organs (Sawa et al. (1999) Genes Dev 13:1079-1088), whereas crc mutants have nectaries that fail to develop and abnormal carpels (Alvarez and Smyth (1999) Development 126:2377-2386). FIL and CRC encode transcription factors containing a zinc finger and a helix-loop-helix similar to the first two helices of the HMG box known to bind DNA (Sawa et al. (1999) Genes Dev 13:1079-1088; Bowman and Smyth (1999) Development 126:2387-2396). FIL expression is restricted to abaxial tissues while CRC expression extends slightly beyond abaxial tissues, being found also in cells adjacent to the presumptive placental positions in developing carpels (Eshed et al. (1999) Cell 99:199-209).
There is a great deal of interest in identifying the genes that encode transcription factors involved in flower development and meristem formation and activity. These genes may be used to engineer plant development and consequently improve yield. Accordingly, the availability of nucleic acid sequences encoding all or a portion of YABBY transcription factors would facilitate studies to better understand the role of said transcription factors in flower and meristem development, to define their gene targets in the whole process, to reconstruct their evolution and analyze phylogenetic relationships, and could provide genetic tools to enhance plant productivity.
Plant Multiprotein Bridging Factors
In eukaryotes transcription initiation requires the action of several proteins acting in concert to initiate mRNA production. Two cis-acting regions of DNA have been identified that bind transcription initiation proteins. The first binding site located approximately 25-30 bp upstream of the transcription initiation site is termed the TATA box. The second region of DNA required for transcription initiation is the upstream activation site (UAS) or enhancer region. This region of DNA is somewhat distal from the TATA box. During transcription initiation RNA polymerase II is directed to the TATA box by general transcription factors. Transcription activators which have both a DNA binding domain and an activation domain bind to the UAS region and stimulate transcription initiation by physically interacting with the general transcription factors and RNA polymerase. Direct physical interactions have been demonstrated between activators and general transcription factors in vitro, such as between the acidic activation domain of herpes simplex virus VP16 and TATA-binding protein (TBP), TFIIB, or TFIIH (Triezenberg et al. (1988) Gene Dev. 2:718-729; Stringer et al. (1990) Nature 345:783-786; Lin et al. (1991) Nature 353:569-571; Xiao et al. (1994) Mol. Cell. Biol. 14:7013-7024).
A third factor that is involved in transcription initiation is the coactivator protein. It is thought that coactivator proteins serve to mediate the interaction between transcriptional activators and general transcription factors. Functional and physical interactions have also been demonstrated between the activators and various transcription coactivators. These transcription coactivators normally can not bind to DNA directly, however they can “bridge” the interaction between transcription activators and general transcription factors (Pugh and Tjian (1990) Cell 61:1187-1197; Kelleher et al. (1990) Cell 61:1209-1215; Berger et al. (1990) Cell 61:1199-1208). One such “bridging” protein identified in Drosophila is the multiprotein bridging factor 1 (MBF1) transcriptional cofactor (Takemaru et al., (1997) PNAS 94(14):7251-7256). This protein has been shown to act as a transcriptional cofactor that interacts with the TATA-binding protein and nuclear hormone receptor FTZ-F1 in Drosophila. 
Accordingly, the availability of nucleic acid sequences encoding all or a portion of plant MBF1 transcription coactivator proteins would facilitate studies to better understand transcription initiation in plants and ultimately provide methods to engineer mechanisms to control transcription.